Internal and external messengers have been studied which code for aspects of growth, development and function of normal retinal cells and also which affect differentiation of retinoblastoma cells in culture. Insulin receptors are present in high concentration in normal retinal cells and in retinoblastoma cells indicating a role for insulin in retinal function. Also, extracellular matrix components such as laminin have been found to affect differentiation of cultured human retinoblastoma cells.